In my opinion you absolutely need a glass lid. If you want to put a canopy on top of the glass lid, that will make the tank look more like a piece of furniture, and is generally a good idea. But first and foremost, you need a glass lid. A canopy without a glass lid is simply bad news.
Why do you need a glass lid? A glass lid reduces evaporation. Evaporation cools the tank down, and by reducing it you save heating costs. With a glass lid, almost all water that evaporates from the tank condenses on the glass lid, and drops right back into the tank. That's the way you want it to be. Removing water from the tank should occur through regular water changes, because water changes do not only remove water, they remove all harmful compounds in the water with it. Evaporation removes only clean water, and wastes energy.
If you have a canopy on an open tank, that canopy will be exposed to excessive moisture, and no wooden canopy will withstand that for long. I should post pictures of the stand of my 240G, where I neglected to close the sumps. There has never been condensation on the wood, because I have good air circulation going through the cabinet, but the wood has darkened on the outside under the varnish, and my wife keeps bugging me daily to do something about it. Next time I set up a tank like that, I'll have glass lids not only on the tank, but also on the sump!
Excessive moisture in a canopy is also bad news for any electrics, like the lights. Corrosion will be phenomenal. Plus if the fish splash, you can get water on the lamps. Those that get hot can break instantly when splashed with cold water, and without a glass lid, the debris will end up in your tank. Even lights that don't get so hot will have a reduced lifespan when exposed to splash-water or condensation-water.
Last but not least, glass lids prevent jumpers. If you have a canopy without glass lids, be prepared to find fish on any horizontal surface within the canopy, such as the lamps. Only with a tight fitting glass lid you can successfully prevent jumpers, and only if you had a tank without glass lids will you know just how much fish jump.
In 30 years of fish keeping, I have broken one glass lid, and it was easily replaced. However, if you are worried about breaking glass lids, consider transparent polyethylene instead of glass. Transparent sheets of polyethylene are not cheap, but unlike glass they are virtually unbreakable. They also don't warp when used as fish tank lids, which is what makes acrylic unsuitable for that purpose.
In summary, I am all for aquarium canopies, but they are a useful addition to a tight fitting glass lids, not a replacement!
